linux-kernel - Re: [PATCH] rust: elaborate safety requirements for `AlwaysReferenceCounted`

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Message-ID: <aBS0ZP9tFnujvodq@google.com>
Date: Fri, 2 May 2025 12:02:44 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Oliver Mangold <oliver.mangold@...me>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: elaborate safety requirements for `AlwaysReferenceCounted`

On Fri, May 02, 2025 at 01:53:57PM +0200, Andreas Hindborg wrote:
> Clarify that implementers of `AlwaysReferenceCounted` must prevent the
> implementer from being directly initialized by users.
> 
> It is a violation of the safety requirements of `AlwaysReferenceCounted` if
> its implementers can be initialized on the stack by users. Although this
> follows from the safety requirements, it is not immediately obvious.
> 
> The following example demonstrates the issue. Note that the safety
> requirements for implementing `AlwaysRefCounted` and for calling
> `ARef::from_raw` are satisfied.
> 
>   struct Empty {}
> 
>   unsafe impl AlwaysRefCounted for Empty {
>       fn inc_ref(&self) {}
>       unsafe fn dec_ref(_obj: NonNull<Self>) {}
>   }
> 
>   fn unsound() -> ARef<Empty> {
>       use core::ptr::NonNull;
>       use kernel::types::{ARef, RefCounted};
> 
>       let mut data = Empty {};
>       let ptr = NonNull::<Empty>::new(&mut data).unwrap();
>       let aref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
> 
>       aref
>   }

I don't think it's entirely impossible to write an AlwaysRefCounted
value that can be on the stack. The type just needs a lifetime
parameter. For example, this API is not unsound:

struct MyDataStorage {
    // ...
}

impl MyDataStorage {
    fn as_aref(&self) -> ARef<MyData<'_>> {
        unsafe { ARef::from_raw(ptr::from_ref(self).cast()) }
    }
}

#[repr(transparent)]
struct MyData<'s> {
    storage: MyDataStorage,
    _lifetime: PhantomData<&'s MyDataStorage>,
}

unsafe impl AlwaysRefCounted for MyData<'_> {
    fn inc_ref(&self) {}
    unsafe fn dec_ref(_obj: NonNull<Self>) {}
}

impl Deref for MyData<'_> {
    type Target = MyDataStorage;
    fn deref(&self) -> &MyDataStorage {
        &self.storage
    }
}

Alice